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Abstract:
Until now, the processes involved in secondary ice production which generate high concentrations of ice crystals in clouds have been poorly understood. However, collisions that involve rimed ice particles or crystal aggregates have the potential to effectively produce secondary ice from their fragmentation. Unfortunately, there have only been a few laboratory studies on ice-ice collision, resulting in an inaccurate representation of this process in microphysical schemes. To address this issue, experiments were conducted at the Wind tunnel laboratory of the Johannes Gutenberg University, Mainz on graupel- graupel and graupel-snowflake collisions under still air conditions at -15 °C and over water saturation. All fragments resulting from graupel-graupel collisions were collected and investigated by means of a digital optical microscope, while fragments from graupel-snowflake collisions were observed and recorded instantly after collision using a holographic instrument. From these experiments, distributions were obtained for fragment sizes, cross sectional areas and aspect ratios. The results showed a higher number of fragments at lower kinetic energy compared to those presented in the literature. 150 to 600 fragments were observed for graupel-graupel collisions, and 70 to 500 fragments for graupel-snowflake collisions between 10−7 and 10−5 J. Parametrizations for fragment size distributions are provided with a mode at 75 µm for graupel-graupel collisions and at 400 µm for graupel-snowflake collisions. These results can be used to improve the representation of ice collision breakup in microphysical schemes.
